That might just be because he's an important character? Main characters and the like don't usually get glossary entries... 
Nevermind, Soter is with the Ordeal at Golgotterath during TUC, he cannot be Kosoter.
But if Kosoter is a Ciphrang or something like that it makes sense to give him a name that could pass for Ainoni (there is also a Kusjeter who dies in TWP) and a name similar to that of people who actually fought in the Holy War.
Anyway, there was clearly something going on in Ainon during 4121.